Frequency selective components are important for many electronic products requiring stable frequency signals or ability to discriminate between signals based on frequency diversity. These functions are difficult to reliably and repeatably realize in monolithic form together with other microelectronic components such as transistors, diodes and the like.
One approach to realizing frequency selective functions employs a mass allowed to vibrate in one or more dimensions (e.g., a pendulum). Such a mass is conveniently realized as a thin membrane supported at critical points, for example, peripherally or alternatively along one edge or end, forming a thin resonator structure. Such structures provide clearly defined mechanical resonances having significant utility, for example, as filters and as frequency stabilizing feedback elements in oscillator circuits. These structures have the advantages of being extremely compact and of providing narrow bandwidth (i.e., high quality factor) frequency selection components that are light weight and which do not require adjustment over the life of the component.
Thin film resonators incorporate a thin film, free-standing membrane. Typically, this is effected by forming a sacrificial layer followed by deposition of the membrane. The sacrificial layer is then selectively removed, leaving a self-supporting layer.
An alternative approach involves forming a cantilevered beam capacitively coupled to adjacent structures (e.g., a conductor placed beneath the beam). The beam is free to vibrate and has one or more resonance frequencies. Disadvantages of these approaches include need to form free-standing structures and also a tendency of the beam to "stick" to adjacent structures if or when the beam comes into contact therewith.
Problems encountered with such devices include reduced Q or quality factor due to at least two causes: (i) reduced quality factor of materials employed for the piezoelectric element and (ii) reduced quality factor for the composite resonator owing to the contributions of the metallizations forming the electrodes. Additionally, higher coupling piezoelectric materials (e.g., LiNbO.sub.3, LiTaO.sub.3, lithium tetraborate, AlPO.sub.4, BiGeO.sub.20, BiSiO.sub.20 and the like) are preferred for some applications but tend to be more difficult to realize in thin film form, especially as oriented films exhibiting significant piezoelectricity.
The Q of the material(s) employed in the resonator may preclude providing the required bandwidth and insertion loss in the completed structure. Generally, narrow bandwidths require high Q materials. Deposited thin-film layers of piezoelectric materials tend to have poorer (i.e., lower) quality factors than the same materials prepared by other techniques (e.g., single-crystal materials) and this may limit the achievable bandwidth. Additionally, employing lossy materials for electrodes (e.g., Au, Ag, Pb etc.) reduces the overall Q of the resonator structure while use of low acoustic loss materials (e.g., Al and alloys thereof) has less of an adverse impact on the Q of the resonator structure. Accordingly, the bandwidth requirements for some applications may preclude use of some materials in the resonator and may require the use of other materials or particular material preparation techniques.
Many applications require robust, light-weight devices to be realized in small form factor and to consume as little electrical power as possible while operating over a broad range of temperatures. For example, satellite communications apparatus have stringent power requirements and also must operate over a broad temperature range. This example also places a premium on size, weight and reliability.
What are needed are apparatus and methods for forming apparatus wherein the apparatus provides a small, light-weight and robust resonator having solid mechanical support and including high quality factor, narrow-bandwidth frequency selection characteristics together with low power consumption requirements and low insertion loss.